Particularly in satellites and base transceiver stations of a terrestrial mobile communications network, but also increasingly in handheld portable devices themselves, multiple antenna radiator elements for communicating over different frequency bandwidths are used. These devices often communicate over disparate frequency bands simultaneously. To conserve space and weight, multiple antennas are sometimes deployed in an organized array of like antenna radiator elements.
Typically, base station antennas are re-configurable in order to adapt to different environments. Re-configurable antennas can save operators and manufacturers substantial amounts of money in smaller inventory requirements. Normally, a large set of antennas that have different beamwidths and gain values is required. A re-configurable antenna can be set either manually prior to mounting, or electrically while in the mast. Smart antennas or adaptive antennas have even more requirements, since they are required to generate complex radiation patterns that have maxima and minima in certain directions. These antennas use phased array techniques to synthesize the required beam.
That the radiating elements communicate simultaneously over different frequency bands raises the specter of mutual coupling between the antenna elements that can degrade the performance of each, which can become a serious problem in smart base station antennas using phased array techniques. Mutual interference among various antenna radiating elements degrades the array's directivity, can de-tune the elements, and creates blind spots (i.e., directions into which the main beam can not be steered). If the mutual coupling is not below a certain level, depending on the application, the array performance may be compromised.
It is well known that mutual coupling may be reduced by increasing physical spacing between the antenna radiating elements, resulting in increased antenna size for the array. See for example C. A. Balanis, “ANTENNA THEORY: ANALYSIS AND DESIGN” (John Wiley and Sons, Inc., 2d ed., 1997). Such increased separation between radiating elements also causes increased sidelobe levels in the radiation pattern. A normal separation of close to a half wavelength results in mutual coupling levels close to about −20 dB. Certain more advanced methods to reduce mutual coupling are listed below.
One approach to reduce mutual coupling among antenna elements is to select substrate materials so as to minimize surface waves. For example, a study done by F. Rostan, E. Heindrich, W. Wiesbeck, entitled “HIGH-PERFORMANCE C-BAND MICROSTRIP PATCH SUBARRAY WITH DUAL POLARIZATION CAPABILITIES”, (PIERS '94, pp. 1-4), compares Duroid and Rohacell substrates at 5.3 GHz. The low permittivity (εr=1.15) Rohacell substrate does not support surface waves and mutual coupling is close to −30 dB, the drawback being that antennas become large. With the higher permittivity (δr=2.2) Duroid substrate the mutual coupling is at about a −23 dB level.
Another approach is to use interference effects to eliminate mutual coupling. H. Wong, K. L. Lau, K. M. Luk, “DESIGN OF DUAL-POLARIZED L-PROBE PATCH ANTENNA ARRAYS WITH HIGH ISOLATION”, IEEE Trans. Ant. Propag., Vol. 52, No. 1, January 2004, pp. 45-52, and L. D. Bamford, J. R. James, A. F. Frey, “MINIMISING MUTUAL COUPLING IN THICK SUBSTRATE MICROSTRIP ANTENNA ARRAYS”, Electronics Letters, Vol. 33, No. 8, 10 Apr., 1997, pp. 648-650, indicate that this approach may be appropriate under some circumstances. The interfering components can be the surface wave in the substrate and the space wave in the air between the antennas. This technique is inherently narrowband, but mutual coupling levels of about −45 dB can be achieved.
Structural modifications of an antenna array can be applied to reduce mutual coupling. These include individual shielding of the antenna elements as in the paper by H. Wong et al. above, ground plane corrugations, using gridded patches for orthogonality, cavity backing of antenna elements, and the use of cuts in the substrate or in the groundplane. The expected mutual coupling levels by using these techniques are between about −25 to about −30 dB.
The use of photonic bandgap (PBG) materials in the ground plane may also be used to reduce mutual coupling. The use of PBG patches in a common ground plane of an antenna array has been reported at higher frequencies (e.g., 5.8 GHz), but the inventors are unaware of work showing that this technique would be operative for typical mobile telephony/cellular communication frequencies (e.g., 2 GHz and lower, especially the UMTS range 1.92-2.17 GHz and the GSM ranges 0.824-0.960 GHz and 1.710-1.990 GHz.). The problem has typically been that the commonly known PBG structures, like mushroom-PBG and uniplanar UC-PBG, are too large in size at low microwave frequencies.